Communications systems with control systems whose bandwidth is switchable are known. Examples of such control systems in communications systems relate to offset compensation, amplitude control and duty cycle control. In this case, different bandwidths which directly adjoin one another with respect to time are often required in different modes. By way of example, it may be expedient to provide a first bandwidth during the transient process of a system under consideration and a second bandwidth after a transient process.
In this case, the intention is to ensure that switching over the bandwidth of the control system generates as few disturbances as possible due to the change in the bandwidth.
In order to switch over the bandwidth in control systems, it is known to switch the bandwidth of an integrating element of the control system. The integrating element is generally realized by a current source, which charges a capacitance. Its bandwidth can be influenced in a simple manner without having to intervene in the actual signal processing parts in the process. Two methods are known for influencing the bandwidth of an integrating element.
Firstly, the bandwidth of an integrating element can be switched over by changing the current which charges or discharges the integrating element. However, switching over the current disadvantageously leads, in the individual circuit parts, to a great change in the operating points and thus to a great disturbance to the control loop.
Furthermore, it is known to change or to switch the size of the capacitance in order to change the bandwidth of an integrating element of a control loop. This does not lead to problems when increasing the bandwidth, i.e. switching out capacitance. However, when reducing the bandwidth, i.e. when supplementarily connecting capacitance, the problem arises that the capacitance that is to be supplementarily connected is not charged to the same voltage as the capacitance already present. This has the effect that a charge equalization between the capacitances is effected at the connection instant. However, the voltage across the capacitance thus disadvantageously changes at the switchover instant, so that a disturbance to the control loop is once again produced.
For the reception and for the regeneration of optical signals after passing through a transmission system, digital optical receivers are known which comprise an analog input part, in which the optical signal is converted into an analog electrical signal, and a digital signal processing part in which the analog signal is regenerated into a digital data signal with a normalized amplitude and clock information. In the analog part, the received optical signal is converted into a photocurrent by means of a photodiode and amplified in a preamplifier.